1. Field of the Invention
The present invention relates to a digital/analog (D/A) converter for converting a digital signal into an analog signal, and to an image processing apparatus for processing an image signal including a half tone image signal by using such a D/A converter.
2. Related Background Art
A dither method using a threshold value matrix, a dot pattern method, and the like are widely used for binarizing an image including a half tone image. However, such a conventional method is associated with a problem that binarization of an image, particularly a dot image, results in deterioration of an image quality because a large amount of moire patterns are generated by the space between dots and a threshold value matrix each having a periodical structure. Furthermore, not only a dot image but also a line image such as of a handwritten character has stepwise unevenness at its edge portion.
An image processing apparatus for solving such problems has been proposed as U.S. Pat. No. 4,926,248. This apparatus uses expensive circuit components such as digital comparators operating at a high speed and has a complicated structure. Other related arts include U.S. Pat. No. 4,800,442 and U.S. Pat. No. 4,926,268. According to these related arts, an image processing apparatus comprises input means for inputting a digital multi-value pixel data, D/A converter means for converting the inputted digital multi-value pixel data into an analog data, pattern signal generator means for generating a predetermined analog pattern signal, and binarizing means for generating a binary pulse signal (PWM signal) having a width corresponding to the value of the digital multi-value pixel data by comparing the analog signal converted by the D/A converter means with the pattern signal generated by the pattern signal generator means.
The shorter the time for establishing an analog output, the more a D/A converter becomes expensive. In order to precisely compare an analog output with an analog pattern signal, an analog signal should be established for a short time. Therefore, it is necessary to use a D/A converter which operates at a speed sufficiently faster than a clock signal for a digital signal, thereby resulting in a high cost.
Specifically, with most conventional D/A converters, the timing when an analog output signal is established is delayed by a certain time from the clock timing for a digital multi-value pixel data. Therefore, it is impossible to use the analog output signal synchronously with the digital input signal.
It is an object of the present invention to solve the above problems.
It is another object of the present invention to provide a D/A converter capable of highly precisely outputting an analog signal at the same clock timing for an input digital signal.
It is a further object of the present invention to provide an image processing apparatus capable of reproducing a high quality image.
The other objects, advantages and effects of the present invention will become more apparent from the following description when read in conjunction with the accompanying drawings.